So did they also find out what was causing the vibration? A worn ball joint isn’t going to cause a vibration. It isn’t a moving part, generally, when simplying driving straight. It is a pivot point for the tires on the top and bottom. A worn joint will cause extreme tire wear that in turn will cause vibration. But in and of itself a loose ball joint will not cause a vibration.
For anyone to pick out a specific mileage and say that “this part” will last “this long” is no more accurate than flipping a coin to see if your pregnant. Mechanical, man made, parts fail whenever the time is right. For an average city dweller that uses his truck as a car 50,000 seems a little early for a 97 and newer F-150. The same truck that is used as a truck on rougher roads, 50,000 miles isn’t hard to believe.
The greasable versus non greasable debate is an interesting one. Personally I like the greasable design but understand why Ford, and all other brands, switched to non greasable joints. First is price. It is simply cheaper to produce a greaseless joint as there is less steps involved to get to the final product. Second is joint longevity. For anyone that owns a 96 or older F150 knows, greasable joints are very short lived. I made my bread and butter on them babies weekly! You could almost smell the money roll in the door if that truck was over 45k miles. The problem was, as Ford described, is a greasable joint is almost never greased properly. For whatever reason people like to pump grease in until it is hanging out all over. This causes a lot of little “seams” for grit, grime, and water to infiltrate the joint by. A properly greased joint only gets 2-3 pumps each service. Same for u-joints.
Since Ford has switched designs (97 and newer) their warranty failure rate has nearly dissappeared. Prior to 97 there was nearly a 50% rate of failure before the warranty expired. That reflected trucks with at least one ball joint or u-joint failure. That’s right, nearly 50% of the trucks had at least one joint fail before 36k miles! That percentage is now below 8% for comparison.
As for taking a ball joint out of the truck and showing you it is loose, good luck. Unless the joint is very nearly going to fall apart you will not show someone the looseness in your hand. Just isn’t enough leverage to do so.
For information a full ball joint replacement job on an F series truck is a flate rate time of 5.9 hours including wheel alignment. That is for all four joints. FWIW the last several months, even a year, at most shops has been very tough, IE slow. Use the knowledge of labor time to negotiate a better price on replacement. A skilled technican can complete the job in roughly half the time, even faster if the end of the day is nearing. 
As a bit of first hand experience I’ll compare my F-150 (I have two 94 and 95) to my folks 97. They have 130,000 miles on their truck and have no ball joint play as of last oil change. My 94 has 165,000 miles on it and has one lower and one upper (opposite sides) that are loose. This will be the third time replacing these joints and they are greasable everytime. Granted a shoemakers kids tend to wear the worst shoes in town, I know better but don’t always do better. 